Zinc compounds, zinc clays and other additives can be used in rubber compounding (e.g., in the manufacture of tires) to obtain desired properties in a finished rubber product. For example, when a rubber product is mixed and formed (e.g., in a mold), it is typically heated in a curing process (i.e., for a length of time and/or at a temperature sufficient to effect such curing). Careful control of process conditions in such a curing process is essential to impart one or more desired characteristics to a cured rubber product. However, in the case of large and/or thick rubber products (e.g., tires), heat transfer to the rubber product during curing may be non-uniform throughout the rubber material due to the heat transfer characteristics of the material being cured (i.e., longer times are necessary for heat transfer to the center of the material from the surface of a mold). This may result in an uneven curing process (e.g., over-cured surface regions relative to the center of the material), yielding a product with undesirable characteristics resulting from such over-curing at a surface of a rubber product, a phenomenon commonly referred to as reversion. Such an over-cured surface region may have a lower hardness, elastic modulus, abrasion resistance, etc. There are a number of products available to correct this problem, such as Perkalink 900 (Flexsys NV, Belgium).
There have been efforts to use zinc montmorillonite clay to replace zinc oxide in rubber compounding to reduce the amount of zinc oxide used in such rubber compounds for both environmental and economic reasons. Such zinc montmorillonite clays are usually made by mixing sodium montmorillonite with zinc chloride to exchange zinc ions with sodium ions in the clay (e.g., as described in PCT/NL2003/000880, the relevant portions of which are incorporated herein by reference). The clay is then washed free of sodium chloride and excess zinc chloride, then dried and milled to form a zinc clay suitable for use as rubber activator. However, such processes typically require a large number of washing steps. In addition, the dried zinc clay product is often extremely hard, and consequently, very difficult to mill into a fine powder suitable for use in rubber compounding. Furthermore, the zinc content of clays produced by such conventional processes is usually very low, with a typical maximum zinc content of less than 5%.
Precipitated silica, carbon black, calcium carbonate, and clay are also used in rubber products to reduce cost and/or impart special properties such as lower rolling resistance for tires, higher modulus, etc. Precipitated silica is preferred over carbon black for lower rolling resistance. Calcium carbonate and clay are preferred additives for cost advantages, as they are natural products and typically require less energy to make. However, their use is limited as they may also result in undesired effects such as lower hardness and modulus in certain cases.
Organoclays may also used in rubber and plastic compositions as fillers. These organoclay fillers are usually made from smectic clays such as calcium montmorillonite, sodium montmorillonite, saponites, nontronites, beidellites, hecorites, etc. These minerals in turn are dominant parts of bentonite rock. Such clays may be treated with acid, then washed free of the acid and other soluble matter, then converted to their corresponding sodium salts by treatment with soda ash or caustic soda. In one process, a sodium montmorillonite clay is then reacted with quaternary ammonium chloride salts (e.g., dimethyldi(hydrogenated tallowalkyl) ammonium chloride) or amines (e.g., laurylamine or tallowalkylamine) and their salts to form an organoclay. The quaternary ammonium salt intercalates between platelets and/or layers of the clay, helping to expand the interlayer distance in the clay to about 20 angstroms or more. Such intercalation into the clay platelets results in easier exfoliation of the clay during rubber compounding or plastic processing. The exfoliation aids in dispersal of the nanoscale platelets from the clay into various compositions, and imparts various properties like higher hardness, modulus, heat resistance, etc. to products containing such intercalated clays.